Weed Seed: Centaurea virgata subsp. squarrosa (Squarrose knapweed)

Family

Asteraceae

Common Name

Squarrose knapweed

Regulation

Prohibited Noxious, Class 1 in the Canadian Weed Seeds Order, 2016 under the Seeds Act. All imported and domestic seed must be free of Prohibited Noxious weed seeds.

Distribution

Canadian: Absent from Canada (Brouillet et al. 2016Footnote 1).

Worldwide: Native to temperate Asia and introduced in the western United States and Michigan (USDA-ARS 2016Footnote 2, DiTomaso and Healy 2007Footnote 3).

Duration of life cycle

Perennial

Seed or fruit type

Achene

Identification features

Size

  • Achene length: 2.5 - 3.5 mm
  • Achene width: 1.5 mm
  • Pappus length: 1.0 - 2.5 mm

Shape

  • Achene is oblong with a rounded bottom and flat top, laterally compressed with a small notch near the base

Surface Texture

  • Achene is smooth and dull

Colour

  • Light brown to straw-coloured achene with strong central stripe, numerous thin side stripes and a pale area around the basal notch

Other Features

  • Collar at top of the achene is narrow and yellowish.
  • Notch at the base of the achene is shallow (under 1.0 mm depth).

Habitat and Crop Association

Degraded rangelands, grasslands, open sites, logged areas, and roadsides (Cal-IPC 2016Footnote 4).

General Information

Squarrose knapweed was accidently introduced into California in 1930s with sheep ranching (Howell 1959Footnote 5). This species tolerates a wide range of temperatures and drought conditions, but does not tolerate cultivation or irrigation (Whitson et al. 1996Footnote 6).

Similar species

Spotted knapweed (Centaurea stoebe)

  • Spotted knapweed achenes are a similar size, oblong shape, dark colour, striped pattern and pappus length as squarrose knapweed.
  • Spotted knapweed is narrower (by 0.5 mm), often lighter in colour with a shallower notch (by 0.5 mm) and a narrower pale area around the notch than squarrose knapweed.

Photos

Figure 1 - Squarrose knapweed (Centaurea virgata subsp. squarrosa) achenes
Squarrose knapweed (Centaurea virgata subsp. squarrosa) achenes
Figure 2 - Squarrose knapweed (Centaurea virgata subsp. squarrosa) achene, side view
Squarrose knapweed (Centaurea virgata subsp. squarrosa) achene, side view
Figure 3 - Squarrose knapweed (Centaurea virgata subsp. squarrosa) achene
Squarrose knapweed (Centaurea virgata subsp. squarrosa) achene
Figure 4 - Squarrose knapweed (Centaurea virgata subsp. squarrosa) achene, basal notch
Squarrose knapweed (Centaurea virgata subsp. squarrosa) achene, basal notch

Similar species

Figure 5 - Similar species: Spotted knapweed (Centurea stoebe) achene
Similar species: Spotted knapweed (Centurea stoebe) achene
Figure 6 - Similar species: Spotted knapweed (Centurea stoebe) achenes
Similar species: Spotted knapweed (Centurea stoebe) achenes
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